The present invention relates to a head positioner having an two-stage type actuator and to a disk drive using the head positioner.
As multimedia technology progresses, the market has recently, strongly requested a disk drive with high recording density that speedily moves a head to a target position on a disk-like recording medium to position the head and records and reproduces mass storage video information, sound information, and character information. For achieving the high-speed and high accurate head positioning, a method receives attention in which the positioning of the head is controlled by together operating two driving mechanisms, namely main and auxiliary driving mechanisms.
The main driving mechanism is mainly used for a large movement such as a seek operation or a plurality of track jumps. The main driving mechanism moves a head supporting mechanism, the head, and a head slider by rotating the head supporting mechanism around a shaft mounted to a chassis. The auxiliary driving mechanism is mainly used for high-speed and micro positioning such as tracking or one track jump. The auxiliary driving mechanism is controlled by a piezo element, namely a piezoelectric element, at the tip of the head supporting mechanism and in a narrow movable range but a wide band, and drives the head and the head slider.
The driving mechanism comprising the main driving mechanism and the auxiliary driving mechanism is generally called a two-stage actuator or a piggyback actuator. The main driving mechanism is called a coarse motion actuator, and the auxiliary driving mechanism is called a fine motion actuator.
A conventional disk drive generally employs an actuator using only a voice coil motor (VCM) as a means for moving a head. It is difficult that the actuator for moving the head across several thousands or tens of thousands of tracks is used for positioning the head with high accuracy, for example, one-tenth or less of one track. Therefore, a head positioner using a two-stage actuator is proposed as a head positioning means (for example, Japanese Patent Application Non-examined Publication No. H10-255418). In this two-stage actuator, as discussed above, a piezoelectric actuator for finely driving a piezo element with control voltage or an electromagnetic micro actuator having a structure similar to the VCM, is disposed as a fine motion actuator on a head supporting mechanism driven by the coarse motion actuator employing the VCM as a driving means. A two-stage actuator controlling method that uses both a piezoelectric effect and an inverse piezoelectric effect of a piezo element to allow the high-speed and high accurate positioning control is also proposed (for example, Japanese Patent Application Non-examined Publication No. S60-35383). In the latter method, a piezo element, namely a piezoelectric element, as the fine motion actuator is disposed on the head supporting mechanism driven by the coarse motion actuator.
Recently, a small disk drive rapidly becomes widespread as an electronic instrument is downsized to become portable, so that an actuator used for the disk drive is downsized. A method of realizing a small fine motion actuator using a piezo element formed of piezoelectric film is also proposed.
Referring to FIG. 7, there is shown an illustration of a conventional fine motion actuator using a piezo element having piezoelectric films. One end of fine motion actuator 90 is fixed to a tip of head supporting mechanism 99 in FIG. 7. The piezo element has two piezoelectric films 92, 93 on both sides of central material 91. Electrodes 92a, 93a are formed and plated on respective outsides of piezoelectric films 92, 93, and polarized in arbitrary directions. The piezo element has this structure and operates as fine motion actuator 90. When voltage is applied to the electrodes 92a, 93a of the piezo element, piezoelectric film 93 expands and piezoelectric film 92 contracts. The contraction and the expansion of respective piezoelectric films 92, 93 cause the piezo element to bend as shown in FIG. 7. For example, when a head is disposed at the tip of the piezo element and displaced by applying the voltage, the head at the tip of the piezo element can be moved. Fine motion actuator 90 can thus control the positioning of the head by this operation.
The thinner the piezoelectric films are, the higher electric field the piezo element obtains, at the same voltage. Piezoelectric films 92, 93 are preferably formed as thinly as possible. While, the thinner piezoelectric films 92, 93 are, the lower mechanical strengths and electric insulation characteristics of them are. The thickness of piezoelectric films 92, 93 are determined after comprehensively evaluating the distortion characteristics, mechanical strengths, and insulation characteristics. Piezoelectric films 92, 93 formed in a thin film shape are extremely fragile and are apt to suffer a defect such as a crack depending on a handling manner of them. It is therefore extremely important to detect the defect occurring in piezoelectric films 92, 93. The surface observation with a metallurgical microscope or the like is very difficult for detecting the defect such as a crack occurring in piezoelectric films 92, 93, so that an ultrasonic inspection meter is used. A method of detecting current generated by the displacement of the piezo element, displaying a result of the detection, and then finding the abnormality is also proposed (for example, Japanese Patent Application Non-examined Publication No. H11-354852).
When an impurity gets mixed with an piezoelectric film in a manufacturing stage of the piezo element, such a defect is apt to occur in the mixing part of the impurity. The defective part generated by the mixing of the impurity disadvantageously causes a crack and the degradation of an electric breakdown characteristic. When the electric breakdown characteristic degrades, the piezo element cannot sufficiently expand or construct, positioning accuracy obtained as the fine motion actuator decreases, and a positioning operation is disabled. Therefore, for preventing the degradation of electric breakdown characteristic by mixing of the impurity or the like, a method of removing a part corresponding to the defective part of the piezoelectric film in an inspecting stage is proposed. This method prevents voltage from locally being applied to the defective part(for example, Japanese Patent Application Non-examined Publication No. 2001-88310).
The method of detecting the defect in the fine motion actuator as shown in the conventional example and the method of removing the defective part by electric breakdown, however, are performed in the inspecting stage of the piezo element. A potential defect that potentially exists in the piezoelectric film and does not become obvious in the inspecting stage cannot be prevented from remaining in the inspecting stage. When a piezo element having the potential defect is used as the fine motion actuator, disadvantageously, a positioning operation of the fine motion actuator itself accelerates degradation by the defective part to degrade a displacement characteristic in a short time.
A defect that does not initially become obvious in the piezoelectric film cannot be detected in the inspecting stage. The method employing the piezo element for the positioning operation by the fine motion actuator uses mechanical distortion, so that the piezo element itself frequently suffers mechanical stress in the operation. When the piezo element itself suffers the mechanical stress, deformation such as increase of a defective part generated by an impurity progresses. When this process is repeated, a defect that does not initially become obvious can become obvious in the form of the degradation of the displacement characteristic. In other words, even when the piezo element is sound in the inspecting stage, the repeating of the displacement of the piezo element itself accelerates the degradation of the characteristic by a defective part such as an impurity, and reduces its life comparing with the life of a sound element.
The fine motion actuator employing the piezo element uses the mechanical distortion differently from an electronic component such as a capacitor. The repeating of the displacement of the piezo element itself accelerates the degradation of the characteristic by a defect that cannot be detected in the initial stage. Therefore, the degradation of the displacement characteristic, for example reduction of capacitance or electric resistance by the defective part in the film, and the breakdown of the piezo element can occur. When the fine motion actuator constituting a head positioner uses a piezo element having a potential defect, disadvantageously, positioning accuracy decreases in a short time, or the positioning operation is disabled.
The present invention addresses the problem discussed above. The present invention aims to provide a head positioner that detects degradation of the performance of a fine motion actuator by a defective part of a piezo element, allows the recovery of the degraded performance when the degradation is detected, and thus realizes stable positioning control for a long time. The present invention also aims to provide a disk drive using the head positioner.
The head positioner of the present invention has, as a head positioning means, an actuator comprising a fine motion actuator unit for performing fine positioning by the piezo element and a coarse motion actuator unit for performing coarse positioning. The head positioner further comprises a degradation detecting means and a performance recovering means. The degradation detecting means measures an electric characteristic of the piezo element and detects the existence of the degradation of the electric characteristic to detect the degradation of the performance of the fine motion actuator unit by the defective part of the piezo element. The performance recovering means, when the degradation of the electric characteristic of the piezo element is detected, applies high voltage to the piezo element to recover the performance of the fine motion actuator unit degraded by the defective part of the piezo element.
In this structure, even when the piezo element constituting the fine motion actuator unit suffers the degradation of a displacement characteristic while being built in the head positioner, the degradation detecting means detects the degradation of the displacement characteristic in the form of that of the electric characteristic. When the degradation of the electric characteristic occurs, the performance recovering means applies high voltage to the piezo element, thereby recovering the degraded performance of the fine motion actuator unit. The degradation detecting means measures the electric characteristic of the fine motion actuator unit at a predetermined time such as a starting time of the head positioner. The degradation detecting means, based on the measuring result, further detects the existence of the degradation of the piezo element constituting the fine motion actuator unit. When the degradation is detected, the high voltage generated by the performance recovering means is applied to the piezo element for a predetermined time. When the degradation is caused by an electric short circuit or the degradation of an electric breakdown characteristic in the piezo element, the applied high voltage can remove the cause. The head positioner of the present invention detects performance degradation of the fine motion actuator unit caused by a potential defect of the piezo element, and allows the recovery of the degraded performance when the degradation is detected. The head positioner can therefore realize the stable positioning control for a long time.
In the head positioner of the present invention, the degradation detecting means measures current flowing through the piezo element as the electric characteristic, and, when the current exceeds a normal current range, determines the performance degradation of the fine motion actuator unit by a defective part of the piezo element.
In this structure, the degradation detecting means measures current flowing through the fine motion actuator unit at a predetermined time such as the starting time of the head positioner. The degradation detecting means, based on the measured current, further detects the existence of the degradation of the piezo element constituting the fine motion actuator unit. When the degradation is detected, high voltage generated by the performance recovering means is applied to the piezo element for a predetermined time. When the degradation is caused by an electric short circuit or the degradation of the electric breakdown characteristic in the piezo element, the applied high voltage can remove the cause. The head positioner of the present invention detects performance degradation of the fine motion actuator unit caused by a potential defect of the piezo element, and allows the recovery of the degraded performance when the degradation is detected. The head positioner can therefore realize the stable positioning control for a long time.
In the head positioner of the present invention, the degradation detecting means measures capacitance of the piezo element as the electric characteristic, and, when the measured capacitance exceeds a normal capacitance range, determines the performance degradation of the fine motion actuator unit by a defective part of the piezo element.
In this structure, the degradation detecting means measures capacitance of the piezo element constituting the fine motion actuator unit at a predetermined time such as the starting time of the head positioner. The degradation detecting means, based on the detected capacitance, further detects the existence of the degradation of the piezo element. When the degradation is detected, the high voltage generated by the performance recovering means is applied to the piezo element for a predetermined time. When the degradation is caused by an electric short circuit or degradation of the electric breakdown characteristic in the piezo element, the applied high voltage can remove the cause. The head positioner of the present invention detects performance degradation of the fine motion actuator unit caused by a potential defect of the piezo element, and allows the recovery of the degraded performance when the degradation is detected. The head positioner can therefore realize the stable positioning control for a long time.
In the head positioner of the present invention, the performance recovering means applies high voltage of alternating current to the piezo element when the performance degradation of the fine motion actuator unit by a defective part of the piezo element is detected.
Comparing with the case that high voltage of direct current is applied to the piezo element, the applying of the high voltage of alternating current reduces stress applied to the piezo element by the high voltage, and can prevent the performance degradation of the fine motion actuator unit caused by application of the high voltage.
In the head positioner of the present invention, the piezo element constituting the fine motion actuator unit is formed of piezoelectric film.
This structure can realize a small fine motion actuator unit and a head positioner allowing stable positioning control for a long time. In the head positioner, the degradation detecting means and the performance recovering means detect the performance degradation of the fine motion actuator unit caused by a potential defect of the piezo element, and can recover the degraded performance when the degradation is detected.
The disk drive of the present invention has the head positioner.
This structure can realize the stable disk drive having a long life that detects the performance degradation of the fine motion actuator unit caused by a potential defect of the piezo element, and can recover the degraded performance when the degradation is detected.